1. Field of the Invention
The present invention relates to low viscosity, alkoxysilane-functional prepolymers, to a process for preparing them and to adhesives, sealants, primers or coatings prepared from these prepolymers.
2. Description of the Prior Art
Alkoxysilane-functional polyurethanes which crosslink via silane polycondensation are well established. A review article on this topic is found for example in “Adhesives Age” 4/1995, page 30 ff. (authors: Ta-Min Feng, B. A. Waldmann). Alkoxysilane-terminated, moisture-curing, one-component polyurethanes are increasingly being used as elastomeric coating, sealing and adhesive compositions in construction and in the automotive industry.
These alkoxysilane-functional polyurethanes can be prepared in accordance with U.S. Pat. No. 3,627,722 or U.S. Pat. No. 3,632,557 by reacting, for example, polyether polyols with an excess of polyisocyanate to form an NCO-containing prepolymer which is then further reacted with an amino-functional alkoxysilane. The resulting alkoxysilane-functional prepolymer contains a high concentration of urea groups and urethane groups, which results in products having high viscosities.
One effective way of reducing at least the portion of the hydrogen bond density resulting from the urea groups is to use secondary aminosilanes to generate substituted ureas. A variety of methods have been proposed for this purpose: U.S. Pat. No. 3,627,722 and U.S. Pat. No. 3,632,557 use alkyl-substituted aminosilanes; U.S. Pat. No. 4,067,844 uses an addition reaction of acrylates with the primary aminosilane; EP-A 596 360 uses an addition reaction of maleic esters with the primary aminosilane; and EP-A 676 403 introduces aryl-substituted aminosilanes. All of these methods, however, are able to replace only one hydrogen atom on the terminal urea group; all other urea protons and urethane protons continue to contribute, via hydrogen bonds, to a high viscosity.
Another appropriate way of reducing the density of the hydrogen bonds and, thus, the viscosity is disclosed in EP-A 372 561, in which very long chain polyether polyols are used with a low level of molecular weight increase through the reaction with the polyisocyanate. This requires polyethers which by virtue of specific preparation processes have a high functionality in conjunction with a low level of unsaturation and polydispersity. Further aspects of this technology are described in WO 99/48942 and WO 00/26271. However, this principle has a significant effect only in the case of very long chain prepolymers, designed for low-modulus binders, and even then it is only possible to eliminate some of the hydrogen bond density.
The possibility of obtaining prepolymers of particularly low viscosity by using isocyanate-functional alkoxysilane units is disclosed inter alia in U.S. Pat. No. 4,345,053. There an OH-functional prepolymer is terminated by an isocyanate-functional alkoxysilane, which theoretically saves one urea group per termination. However, the OH-functional prepolymer still contains urethane groups resulting from the reaction of a polyether polyol with diisocyanate. The amount of these urethane groups, as is also disclosed in EP-A 372 561, can be reduced by using specially prepared long-chain polyethers having a low level of unsaturation and polydispersity. In the case of a stoichiometric reaction of isocyanate-functional alkoxysilane units, binders are obtained which, due to inadequate capping, especially when using very long chain polyethers, are unable to crosslink sufficiently on curing. This leads to very soft polymers having a high surface tack and a deficient resilience, or a high plastic deformability.
It is an object of the present invention to provide modified alkoxysilane-functional prepolymers which do not have these disadvantages of the prior art, but have a comparable viscosity.
It has now been found that prepolymers having the required properties can be prepared by initially reacting long-chain polyethers or OH-functional polyether prepolymers with an excess of isocyanate functional alkoxysilane and removing the excess of isocyanate groups by subsequent allophanatization or reaction with a low molecular weight NCO-reactive compound.